The Substance Which Is Alive in Plants 143 



chapter on Metabolism further illustrates, is this, that proto- 

 plasm, despite its aspect of simplicity, is not a single substance, 

 but a very heterogeneous mixture of many different substances 

 of diverse grades of complexity, from the simplest of mineral 

 salts up to the most complicated of pro- 

 teins. None of these substances, how- 

 ever, are of themselves alive, nor has 

 chemical analysis yet succeeded in lo- 

 cating any distinctively living constit- 

 uent, any protoplasm par excellence, 

 although we are logically bound to be- 

 lieve that some such substance must 

 exist as a seat for the distinctive prop- 

 erties of life. Protoplasm, therefore, is 

 probably composed chemically of two 

 classes of materials; first, a very small 

 amount of a distinctively living constit- 

 uent, not yet identified, but consisting, 

 in the fibers, or else the ground substance 

 of its physical texture ; and second, a very 

 large amount of various non-living sub- 

 stances, nutritive and other, which are 

 under the control of the living constit- 

 uent. 



There are, however, some further 

 chemical facts about protoplasm which FlG . 46 ._ T he protoplasm of a 

 go a little way towards explaining its hair ceil of a Gourd, projected 



against a black background. 

 Various powers. Thus, a part Of its COn- (Reduced from Sachs' Lec- 



stituents (in general the most compli- 

 cated) are very unstable, or, chemically stated, labile, and 

 change their composition under slight provocation whether from 

 without or within. Such changes are accompanied, like all 

 others of a chemical nature, by transformations of energy, either 

 release or absorption. And these in turn cause other changes, 



